Chemical explosives or bombs, sometimes termed improvised explosive devices (IEDs), carried in vehicles, left behind in packages, or delivered on the person of a suicide bomber, present a threat to citizens, structures, and public transportation in the United States and to government and military installations and military personnel outside the U.S. as well. The detection of explosives in vehicles and packages, and especially on the person of suicide bombers, must be very rapid and involve minimal or no contact with the vehicles, packages, or persons. To limit the disruption of public transportation systems, passenger screening devices must minimize false positive alarms. Such screening devices must, therefore, be able to specifically identify threats.
Known systems for screening vehicles, packages, and persons include magnetometers, x-ray devices (including backscatter x-ray devices), and terahertz imaging systems. Magnetometers may be ineffective at detecting an explosive device because the device may have little or no metal content. Additionally, magnetometer screening requires coming in close proximity to the suspect package or person, such that screening personnel may be injured if an explosion occurs.
X-ray devices have been incorporated into “drive by” systems used to inspect vehicles and into personnel scanners used to detect threat objects hidden by clothing. X-ray screening devices are able to reveal hidden threat objects, however these device are only able to determine the shape and density of the hidden object. X-ray screening devices are not able to determine the specific chemical composition of the object. As such, x-ray screening devices may be ineffective at detecting an explosive device. Additionally, x-ray screening typically requires coming in close proximity to the suspect vehicle, package, or person, such that screening personnel may be injured if an explosion occurs.
Terahertz imaging systems use radio waves transmitted at terahertz frequencies (100 gigahertz (GHz) to 10 terahertz (THz)), known as terahertz radiation, to produce an x-ray-like image capable of detecting threat objects, such as those hidden under clothing. Additionally, terahertz imaging systems may be capable of determining the specific chemical composition of an imaged object by comparing the absorption spectra of a suspect object to the absorption spectra of a known threat material. However, screening using known terahertz imaging systems requires coming in close proximity to the suspect vehicle, package or person, which again may cause injury or death to screening personnel if an explosion occurs.
As such, there is a need for an imaging system capable of quickly and accurately detecting threat objects, such as explosive devices, while allowing screening personnel and equipment to maintain a safe distance from the suspect vehicle, package, or person.